Hi,
There was a guy on the Unofficial Humminbird Forum who had a 998 interlinked to a 957. He said he acquired interlink problems when he updated the 998 to 4.97. The interlink problems went away when he backed down to 4.88 on the 998. I have not heard anyone else associate interlink issues to the units hardware revision but you might give it a try.
Take care,
Roger